1. Field of the Invention
The present invention is directed to an agent for desulfurization, dephosphorization, desiliconizing and denitriding of pig-iron, cast iron and similar melts and a process for the treatment thereof.
2. Information Disclosure Statement
In recent decades, many proposals for the desulfurization and dephosphorization of iron melts, chromium and manganese melts have been described in the literature. From these agents, those based on calcium carbide, containing gas-evolving additives, gained wide acceptance. Agents in DE-PS 17 58 520 permit--by employing corresponding technologies--describe the treatment of large volumes of iron in which calcium carbonate is used as a gas-evolving additive. In German application DE PS 22 52 795, hydrated lime is proposed as a gas evolving component. EP-A 0 138 417 describes a mixture composed of burnt lime, lime-stone-carbon and fluospar. These agents exhibit--due to their extremely fine grain size--poor flow performance and can only be fed with pneumatic systems with difficulties. The general use of these so far known agents is limited by the fact that they cause large slag volumes and considerable iron losses which can reach up to 70% of the total slag weight.
These additives have to be considered as inactive with respect to metallurgical reactions; the purpose of this burden-material is to improve the dilution of the basic component--in addition to gas release--the pneumatic feeding. In general, fluid agents are added to improve the flow performance of the base material as described in EPO 0 226 994.
Some of these dilutants release gas which is supposed to improve the mixing of the desulfurizer with the liquid melt. The residue of these components --regardless of whether lime-hydrate or lime-stone is used--is lime, at the temperatures of the melt present in the solid state. Because diffusion controls the desulfurization with solid phases, this process is governed decisively by the residence time of the active particle. The lime particle developed from the dilutants contributes only a limited extent to the reaction with the melt; therefore, it can be considered as passive with respect to the metallurgical reactions.
The slags resulting from mixtures consisting of calcium carbide, lime-stone and fluid aids tend to form a crust. The solidified slag is difficult to remove. Mechanical equipment is necessary to remove this crust. Very often it is necessary to apply fluospar or soda ash after treatment to fluidize this accumulation.
Another effect of these gas evolving additives is consumption of a part of the calcium carbide for reaction with the CO.sub.2 and [OH].sup.- respectively. In DE-PS 22 52 796, additives are described which release hydrogen or nitrogen. The effect of these gases is neutralized to a great extent by the carrier gas. However, the more important problem of solid-liquid-reaction, dominating with calcium carbide, is not solved.
Fluospar was also proposed to liquify the slag, but fluospar causes environmental problems and a metallurgical potential cannot be contributed to this mineral.
Desulfurizing by means of an impeller produces deposits on the side walls of the ladle. As these crusts also contain calcium carbide, it is eliminated from the desulfurizing process. Further the impeller may be destroyed.
Another proposal recommends using a calcium carbide according to a eutectic composition containing some 65-67 w.-% CaC.sub.2 chemically, with the remainder mainly CaO. However, this composition did not change the slag structure which also contained up to 70% by weight of iron granules.
In general it is not possible to make use of the best working conditions, for instance, the lowest lance position or the correct immersion depth for the impeller. These operational factors can reduce the efficiency of the agent considerably. Therefore, there is a permanent demand for a desulfurizing agent which overcomes these shortcomings.
The addition of aluminum metal to the metal melt, in contents up to 0.04% may improve the efficiency of the desulfurizer, but does not eliminate the difficulties with the dry slag, caused by calcium carbide and also by lime.
Another negative effect associated with the use of dry desulfurizing agents such as calcium carbide, is the so-called incubation time, the time during the first 20 to 30% of the whole treatment time in which the desulfurizing reaction is very slow, until the slag is heated to the approximate temperature of the melt.
The additives used so far to blend with the basic desulfurizer have had little or no effect on the inherent problems of a solid-liquid reaction. Moreover these additives contributed only a very limited extent to the reduction and overcoming of operational deficiencies of the desulfurizing process used.